The present invention relates generally to position sensing and more specifically to optically determining focus and lateral (transverse) position. This could be used, for example, to focus and align a printhead with respect to a printing substrate.
Microlens-scanner exposure systems are being developed for high-resolution, maskless lithography applications. (See, for example, U.S. Pat. Nos. 6,133,986, 6,424,404, and 6,498,685.) A microlens-scanner system, uses a microlens array to focus illumination onto an array of diffraction-limited focal spots, which are scanned across a printing surface as the spots are intensity-modulated to synthesize a high-resolution latent image in a photosensitive medium. The spots are modulated by means of a digitally-controlled spatial light modulator (SLM), which may be imaged onto the microlens array by means of projection optics, or may alternatively be incorporated within the microlens array itself.
The microlens arrays and associated Microsystems, such as sensors and micro-positioning actuators, are formed on planar “printhead” structures. The scanning motion may be effected by moving either the printheads or the printing surface, but in either case fine-alignment positioning could be done with position sensors and servomechanisms formed integrally with the printhead.
There are advantages to putting much of the system functionality into the printheads. For example, the projection optics can be eliminated by integrating the SLM into the printhead. With an integrated SLM, the optics external to the printhead need only provide illumination of sufficient uniformity and collimation. (There is no need for stringent alignment tolerances in the illumination optics.) Furthermore, an integrated SLM eliminates optical cross talk between SLM pixels, and also makes it possible to operate multiple printheads in parallel with a single illumination source. There are also benefits to placing the focus and fine-alignment sensors and actuators on the printhead in close proximity to the exposure microlenses. The fine-positioning servomechanism would not need to drive large masses or maintain stringent positioning tolerances between multiple monolithic elements (stages, lenses, etc.) spanning large distances; and the close coupling between all the critical system components—position sensors, actuators, and exposure microlenses—would eliminate tolerance stack-up in the focus/alignment servomechanism, enabling very accurate and responsive position tracking while also simplifying the system.
Maskless lithography with improved focus/alignment control could make the use of multi-level etch masks practical for achieving print resolution far beyond the optical limit. The method is described in U.S. Pat. No. 6,133,986 (see FIGS. 11a–f, 12 and 13 in '986), and is illustrated in FIGS. 1, 2A and 2B herein. FIG. 1 illustrates a semiconductor wafer substrate 101, lower planarization layer 102, lower etch mask layer 103, upper planarization layer 104, and upper etch mask layer 105. The mask layers are lithographically patterned, using high-contrast, high-k1, imaging to obtain accurate edge placement; and the two mask layers are used to etch narrow trenches in the substrate (e.g. trench 106) with trench widths much smaller than the optical resolution limit. FIGS. 2A and 2B illustrate a similar process for forming sub-resolution etched lines. In this example, the upper and lower mask edges overlap and the upper mask is used to selectively remove the middle portion of each lower mask line (e.g., portion 201 is removed from line 202, leaving the portions 203 and 204 shadowed by the upper mask—see FIG. 2A). The lower mask is then used to etch narrow lines in the substrate (e.g. line 205, FIG. 2B). For this type of application the extra cost incurred by the multiple etch masks would be mitigated by using maskless lithography, and printhead-integrated focus/alignment controls could achieve the very stringent mask overlay tolerances that would be required.
Several types of focus/alignment mechanisms are discussed in the prior art. U.S. Pat. No. 6,133,986 describes a parallel-confocal, Moire imaging technique for simultaneously measuring focus and alignment. U.S. Pat. Nos. 6,392,752 and 6,628,390 describe microlens imaging and position-sensing mechanisms that employ optical phase-contrast methods for improved sensitivity. U.S. Pat. No. 6,498,685 mentions the use of capacitance sensors and other types of proximal probes for focus and alignment tracking. Similar systems are employed in commercial position encoder systems—for example, NanoWave Inc. (www.nanowave.com) manufactures optical and capacitance-probe encoders exhibiting subnanometer measurement resolution. These systems employ grating targets, similar to the mechanisms described in the aforementioned patent disclosures.
Alignment tracking is similar, in some respects, to overlay metrology, which is a post-process measurement of alignment error. A technique for measuring overlay by detecting phase contrast in overlapping diffraction gratings is described in “Scatterometry-based overlay metrology,” Proc. SPIE, vol. 5038, 126–137 (2003). This method exhibits subnanometer measurement resolution, an order of magnitude better than imaging-based methods. The method uses gratings formed on the test device with fixed positional relationships, and is therefore not directly applicable to alignment tracking; however it illustrates the kind of measurement capability that is possible with phase-contrast optical detection methods.